UNCLASSIFIED

AD 406 2071

DEFENSE DOCUMENTATION CENTERFOR

SCIENTIFIC AND TECHNICAL INFORMATION

CAMERON STATION. ALEXANDRIA. VIRGINIA

UNCLASSIFIED

NM72: Uem gmovezn or other drawiqnp, spe:l-fications or other data are used flor any parposeother than in connection with a definitely relatedgoverumut procunment operation, the U. B.Oo vernent thereby incurs no responsibility, nor amyobl~iation •iatsoever; and the fact that the Govern-mt my have fozualted., furnished, or In any vaysupplied the mid drawings, specificatimsp or otherdata is not to be regaded by Implication or other-wise a in any mner licensing the holder or anyother person or corporation, or conveying any rihtsor peruission to mnufacture, use or sell anypatented invention that my in any wvay be relatedthereto.

Wright Aeronautical Serial Report No. CTR.00-272

Final Report

ADVANCED HEAT TRANSFER ELEMENTSFOR

LIQUID METAL REGENERATOR SYSTEMS

April 30, 1963

C%4J

g Cm•

I

Copy of 50

Wright Aeronautical Serial Report No. CTR.00-272

4. Final Report

* - ADVANCED HEAT TRANSFER ELEMENTSFOR

LIQUID METAL REGENERATOR SYSTEMS

In Fulfillment of Item 8 Contract NOw 62-0601C

with

Department of the NavyBureau of Naval Weapons

Washington 25, D.C.

April 30, 1963

Written by:

L. R. CoxR. GillF. W. Saco

F. W. Saco, Poject Engineer

Approved:

4 -Sylvester Lombardo, Project Manager

W

SWRIGHT AERONAUITICAL DIVISION * CURTISS-WRIGHT CORPORATION * WOOD-RIDGE. N. U...U S. A.

TABLE OF CONTENTS

Pane No.

I. INTRODUCTION 1-1

I. BACKGROUND 2-1

III. DESCRIPTION OF WORK 3-1

IV. OPTIMIZATION STUDIES 4-1

V. METALLURGICAL STUDIES 5-1

VI. FABRICATION STUDIES 6-1

VII. FABRICATION OF SAMPLE HEAT EXCHANGER 7-1

VIII. RESULTS & DISCUSSION 8-1

IX. REC(MlENDATIONS 9-1

WRIGHT AERONAUTICAL DIVISION * CURTISS-WRIGHT CORPORATION * WOOD-RIDGE. N. J . U S. A,

I INTRODUCTION

This report sumnarizes the work accomplished on the feasibility ofextending the materials and design restrictions of current finnedtube heat transfer elements to provide lighter weight, higher per-formance liquid metal regenerator systems. Under this program thefollowing items were investigated:

(a) Fin Design - The feasibility of fabricating tapered finsand fins with a fin-to-tube diameter ratio greater than2.0:1.

(b) Fin Material - The feasibility of using a material requiringno cladding or protective coating.

(c) Fin Protection - The feasibility of reducing the protectivecladding by means of coatings, platings, etc.

(d) Fin Thickness - The feasibility of reducing the total finthickness to .003".

Metallurgical, analytical, and fabrication studies performed showthat significant improvements in heat exchanger weight can be achievedby the use of finned tubes made with extended surfaces of aluminumbronze. The properties of aluminum bronze which provide improvedperformance heat exchanger surfaces are as follows:

(a) Excellent oxidation resistance, and therefore, cladding isnot required.

(b) Excellent ductility which makes possible the fabrication of

fins with thicknesses of .003" or less.

(c) High conductivity.

Sample finned tube elements made from aluminum bronze fins were de-veloped, and a small heat exchanger core was fabricated to demonstratefeasibility of the method.

On the basis of advanced 4000 H.P. shaft engines, improvements of 20to 307. in core weight of liquid metal regenerator systems is indicatedwith the heat transfer elements developed.

" " IS I

WR•IGHT AERONAuITICAL DIVISION * CURTISS-WRIGHT CORPORATION• * WuU•U-RIDGE N S A

II BACKGROUND

The function of a liquid metal regenerator system for turbopropengines is to reduce fuel consumption to the extent that the addedweight of the system is more than offset by the weight of fuel savedduring the mission of the aircraft. A liquid metal regenerator systemconsists of a heat exchanger to extract heat from the turbine exhaust,a heat exchanger to release heat to the compressor air prior to itsentry to the combustor, and a circulating liquid metal system to conveyheat from the turbine heat exchanger to the compressor heat exchanger.

The heat exchangers are comprised of heat transfer elements joined tocollecting headers. The heat transfer elements are the active portionof the system and constitute the major portion of the weight of thesystem. Improvement in the performance versus weight of these elementswill directly improve the effect of the liquid metal regenerator systemon the engine. Therefore, efforts have been directed towards develop-ing advanced heat transfer elements specifically suited for liquidmetal regenerator service.

The heat transfer elements used to make the heat exchangers in theliquid metal regenerator system, evaluated and developed under aseparate portion of this contract, represent the latest advances ofthe art. These heat transfer elements are finned tubes arranged in aserpentine fashion and are hereby described to serve as a reference forthe work covered under this portion of the contract.

Tube O.D. .188 inchTube Wall .015 inchFin O.D. .375 inchFins per inch 30Total fin thickness .005 inchTube material 316 stainless steelFin material Stainless steel clad high conductivity

copperCopper thickness .0025 inches

The fins are positioned by shallow grooves in the tube and are bondedto the tubes by a nickel base braze. The braze material is sprayedonto the tubes after the fins have been wound. Braze is deposited onthe edge of the fin to protect the exposed edge of the clad copper anddeposited on the tube at the base of the fins to form a thermal bondbetween the fins and the tube. The tubes are bent into serpentinesand the serpentines further bent to suit the heat exchanger configura-tion prior to brazing in controlled atmosphere furnace. A typicalserpentine heat transfer element for an axial flow compressor heatexchanger is shown in Figure 2-1. The plane of the serpentine is bent

2-1

WWRIGHT AERONALuTICAL DIVISION * CURTISS-WRIGHT CORPORATION * WOOD-RIDGE. N. J.. U S A

4

to an involute curve to obtain constant tube spacing throughout theannular cross sectional area of the heat exchanger.

2-2

WRIGHT AERONAUTICAL DIVISION CURTISS-WRIGHT CORPORATION * WOOD-RIDGE. N J . U S. A.

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III DESCRIPTION OF WORK

Efforts to reduce weight and increase performance of finned tube heattransfer elements have been directed to: (1) optimization studies,(2) metallurgical studies, and (3) fabrication studies. These effortshave terminated in the fabrication of a full scale sample heatexchanger, demonstrating feasibility of manufacture of advanced finnedtube heat transfer elements.

Preliminary screening of performance, metallurgical and fabricationcharacteristics led to the selection of coated copper fins andunprotected aluminum bronze fins as the most promising materials forinvestigation. Optimization studies were made on both these finmaterials with reasonable allowances for oxidation protection of thecopper fins. A cursory examination revealed that increasing gains inperformance and weight reduction could be made by going to smaller andsmaller tube and fin diameters. However, these investigations werelimited to diameters considered feasible for heat transfer elementscapable of manufacture and use in the immediate future. Studies weremade for real heat exchangers as applied to a 3750 HP turboprop studyengine rather than by separate treatment of various parameters.

3-1

WRIGHT AERONAUTICAL DIVISION ° CURTISS.WRIGHT CORPORATION - WOOD-RIDGE N U 5 A

IV OPTIMIZATION STUDIES

Finned tube serpentine heat transfer elements are considered mostsuitable for liquid-metal-coupled applications when compared to otherconventional types such as plain tube banks and plate-fin combinationsprimarily because of their: (1) adaptability to approach counterflowconditions; (2) high ratio of gas side surface area to liquid metalsurface area; (3) high ratio of gas side volume to liquid metalvolume; (4) flexibility to accommodate large thermal gradients; and(5) flexibility of arrangement.

While it is true that other types of surfaces can achieve some of theabove advantages, it is felt that the finned tube serpentine is theonly type which can be optimized, independently, in each of the fiveareas enumerated above.

Selection of an optimum core of the finned tube type requires establish-ing the most desirable values for the following four geometric para-meters:

(1) Outside fin diameter(2) Outside tube diameter(3) Fin thickness(4) Number of fins per inch

In order to establish the optimum core configuration for the studyengine and to thereby, generate design goals for mechanical andmetallurgical feasibility studies, an analysis was conducted usingelectronic data processing methods in which heat exchanger coregeometry was systematically varied while maintaining constant heatexchanger inlet conditions and constant heat exchanger performancei.e., overall effectiveness, ratio of component effectiveness, overallpressure drop and ratio of pressure drops.

Heat exchanger performance parameters were selected as:

E0 = 70%

Eh/Ec - 1.0

(ZAP/P)c - .02

(^P/P)h - .02

4-1

WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE N UJ S A

where

E0 - Overall Effectiveness

Eh - Turbine (Hot Side) Heat Exchanger Effectiveness

Ec - Compressor (Cold Side) Heat Exchanger Effectiveness

(6 P/P)c - Compressor Heat Exchanger Pressure Drop Relative toInlet Pressure

(AP/P)h - Turbine Heat Exchanger Pressure Drop Relative to

Inlet Pressure

The parameters investigated were:

(1) Tube O.D.; .1875 in. and .15625 in.(2) Ratio of fin O.D. to tube O.D.; 2.0, 2.5 and 3.0.(3) Overall fin thickness; .0025 in., .003 in., .004 in. and

.005 in.

It had previously been established that the number of fins per inchshould be maintained as high as practical. (TREC Report 61-46,"Liquid Metal Regenerator Feasibility Study for a Light Weight Turbo-shaft Engine".) However, the ability to operate at engine conditionswithout fouling has not been investigated beyond 30 fins per inch.This parameter was, therefore, maintained at 30 fins per inch for allconfigurations. Tube wall thickness was also maintained at .015 inches.

Each combination of these parameters was investigated utilizing twobasically different fin materials: (1) a solid aluminum bronze alloyrequiring no additional surface protection and (2) solid copperhaving .0005 inches of nickel plate on all exposed surfaces. The finthickness in both cases was the overall thickness including the nickelplate in the latter case.

The results of this analysis in terms of core weight versus effective-ness are shown on Figures 4-1 through 4-8.

For comparison, the present state-of-the-art fins (.375 O.D., .005"thick with .0025 clad and 2.0 diameter ratio) are shown (*) on thesame plots as the advanced types.

Since this was an optimization study for fin geometry and would notnecessarily give the same results for both exchangers, the results arepresented separately for the turbine heat exchanger and the compressorheat exchanger. The results indicate the following:

4-2

WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE N I U S A

(1) For the same diameter ratio, fin thickness and tube O.D.,the plated copper fin configuration resulted in lightercore weights than did the aluminum bronze fins for both thecompressor exchanger and the turbine exchanger.

(2) For the same fin thickness and fin material the smaller tubediameter resulted in lower weights for both heat exchangercores.

(3) The minimum thickness investigated resulted in minimum coreweights for each combination of the other parameters. It isfelt, therefore, that fabrication techniques will be thecontrolling factor in this area, rather than heat transferrequirements.

(4) In general, a diameter ratio of 2.0 resulted in lowest coreweight for the compressor heat exchanger and a diameterratio of 2.5 resulted in lowest core weight for the turbineheat exchanger. This was true for both tube diametersinvestigated and for both the plated copper fin and thealuminum-bronze fin.

(5) For the turbine heat exchanger, the thinnest fins investigat-ed appear to be farther from optimum than for the compressorheat exchanger. This results from the lower heat transfercoefficient in the turbine exchanger, which in turn resultsin higher fin efficiencies for the same thickness fin thanin the case of the compressor heat exchanger.

(6) At an overall fin thickness of .005" there is not a signifi-cant advantage for either the aluminum bronze fin or theplated copper fin over the clad copper fin. However, the.005 inch clad copper fin is approaching the limit ofmanufacturing feasibility for this type of configuration,whereas it appears quite feasible at this time to producefins .003 inch thickness of the other two configurations.The .005 inch clad copper fin should not, therefore, becompared to a similar thickness fin, but should be comparedon the basis of similar manufacturing feasibility, to finsof the other configurations having thicknesses of .003 inchesor less. On this basis, either the aluminum bronze fins orthe plated copper fins are clearly superior inasmuch as for.003 inches thickness they result in heat exchanger coreweights which are from 14 to 24 percent lighter thancorresponding core weights for .005 inch thick clad copperfins, tube diameter and wall thickness being the same.

4-3

WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE. N, 0 S A

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WRIGHT AERONAUITICAL DIVISION ,CURTISS-WRIGHT CORPORATION - WOOD-RIDGE. N. J. U. S A.

V METALLURGICAL STUDIES

Current advanced heat transfer elements incorporate a 0.005" x 0.094"stainless clad copper fin with a spacing of 30 fins per inch, producedby spiral winding on a 0.1875" O.D. stainless steel tube (Fig. 5-1).Further reductions in the thickness of this fin material are precludeddue to a lack of ductility in thin sections of this nature and possiblediscontinuities in the protective cladding. Another drawback to theutilization of a clad fin material is the necessity of protecting theedges from the corrosive and oxidizing environments encountered duringengine operation. At present, protection is accomplished by sprayinga nickel base braze material on the wound finned tube, but a weightpenalty is incurred.

A. Materials

The optimum fin material should possess high temperature oxidation andcorrosion resistance combined with an inherent high value of thermalconductivity, approaching that of copper at the intended fin operatingtemperature (600-1100*F). A literature survey indicated aluminumbronze (4 - 6% aluminum) and a zirconium/copper base alloy (Amzirc)were possible choices for an application of this nature. Amzirc hasan excellent value of thermal conductivity (Fig. 5-2), but its oxida-tion resistance was unknown at temperatures above 800*F. The thermalconductivity of aluminum bronze is considerably less than that of purecopper at ambient temperature but the Oak Ridge National Laboratorieshas reported its value increases steadily with increasing temperatureas depicted in Fig. 5-2. Heat exchanger core configuration studiesusing these data and actual engine conditions indicated that an alumin-um bronze finned tube would be comparable, in terms of performance, toa clad copper finned tube. Oak Ridge also indicated that the 6% alum-inum bronze alloy evaluated at the laboratory was oxidation resistantto within 70"C of its melting point. For purposes of obtaining ahigher performance fin material, a 5% aluminum bronze alloy wasselected for evaluation. It was postulated that the oxidation resist-ance of this alloy would be adequate for the intended application andan increase in the thermal conductivity would be realized due to thedecrease in aluminum content.

B. Material Evaluation

Exposure of the promising materials for 100 hours in static air at1200*F was selected as a screening test. The 5% aluminum bronze alloyincurred no harmful effects from oxidation as a result of this exposuretreatment. Additional exposure under identical conditions, for up to500 hours, did not significantly affect the subject alloy. Fig. 5-3depicts a cross-section of the 0.005" thick aluminum bronze foil during

5-1

WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION • WOOD-RIDGE N I u 1, A

various stages of exposure. Recrystallization is evident in the ex-posed sections, but no significant reduction in thickness or inter-granular penetration has occurred. The zirconium copper alloy, on theother hand, sustained a total cross-sectional loss of 0.0045" due tooxidation after 100-hours exposure in static air at 1200*F. The oxideformed did not adhere to the parent metal and therefore, was not pro-tective in nature. Since heat exchanger operating conditions are moresevere than those encountered in the static exposure test, no furthertesting of this alloy was scheduled.

The primary barrier to further reductions in fin thickness is the lackof ductility of the clad materials. Previous experience has dictatedthat a minimum ductility of 28% elongation in six inches is requiredto successfully fabricate finned tubing of the dimensions previouslydescribed, by means of a spiral winding process. Attempts to wind anelement with 0.0035" stainless clad copper fin material were unsuccess-ful due to a lack of ductility of this nature, which resulted in crack-ing and subsequent breaking of the strip during the precoiling andwinding operations. Attempts to heat treat this stock to the requiredductility were unsuccessful. In order to establish the feasibility ofutilizing the 5% aluminum bronze in this fabrication process, the alloywas procured in "ribbon" form (0.003" thick, 3/32" wide). Tensiletests of the material in the as-received condition indicated a duc-tility of 2 - 3% elongation in 6". This low ductility was expected,however, due to the large amounts of mechanical deformation incurredin rolling the material to foil. Subsequent annealing treatmentsraised the ductility of the aluminum bronze ribbon above that requiredfor spiral winding (Table 5-1). Of the three annealing temperaturestested (950, 1200, and 17000F), 1200"F for five minutes in argon pro-duced the highest ductility of 46. elongation in 6".

C. Oxidation Resistant Coatinis

A parallel approach was adopted to establish the feasibility of coatingOFHC copper fins for operation at design temperatures (up to 12000 F).The type of coatings to be evaluated consisted of the following:plated (electrolytic and electroless), vapor deposited, and diffusion.A screening test consisting of exposure at 12000F for 100 hours instatic air was selected for evaluation of feasible coatings. Smallfinned tube samples incorporating 0.005" thick OFHC copper fins, brazedto a stainless steel tube with a fin spacing of approximately thirtyper inch, were fabricated to evaluate the various coatings.

D. Electrolytic Coatings

Several OFHC copper "washers", electrolytically plated with nickel andchromium, were subjected to the screening test to determine the suscept-

5-2

WRIGHT AERONAUTICAL DIVISION " CURTISS-WRIGHT CORPORATION " WOOD-RIDGE N J L; S A

ibility of the coatings to oxidation and cracking under thermal shock.Metallographic examination of the chromium plated samples revealedcracking in the as-plated sample which led to complete coating break-down on the exposed fins (Fig. 5-4). Due to the inherent brittlenessof the electrolytic chromium plate, it is believed that this crackingwas caused during plating or subsequent handling operations. Theelectrolytic nickel plate exhibited sufficient ductility to withstandcracking of this nature, but subsequent exposure treatments resultedin isolated breakdown of the coating (Fig. 5-5). Since plating tech-niques of this type are applicable only to individual fins and not afinal assembly, it is expected that coatings which can be applied toan assembled and brazed heat exchanger serpentine will preclude theiruse.

E. Vapor Deposited Coatings

Fin spacings of thirty fins per inch also severly limit the use ofvapor deposited coatings. The strength and density of coatings vapordeposited on the sides of the fins would not be adequate to provideprotection at heat exchanger operating temperatures for a substi.4ntiallength of time. This lack of strength is due to the poor angle ofincidence at which coatings of this nature would have to be deposited.Coatings of this type, however, would be useful in protecting theedges of a clad fin element, enabling the incorporation of tubes pre-coated with braze alloy in the fabrication process.

F. Electroless Nickel Plating

Several brazed finned tube samples were plated with an electrolessnickel plate to a thickness of 0.0005" (Fig. 5-6A). Metallographicexamination revealed that the plating operation had completely coveredall the exposed surfaces, including the critical tip and base areas(Fig. 5-6B) of the fins. Exposure of similar samples for 100 hoursat 12000F induced localized cracking which resulted in subsequent pen-etration and peeling of the protective nickel plate. Superficial crackswere produced in a sample exposed only 15 minutes at 1200°F (Fig. 5-7).It was postulated that this phenomenon was caused by the inherentstress present in the protective nickel plate, combined with the thermalshock incurred by placing the sample in a hot furnace at 12000F.

The ability to withstand thermal shock is a primary consideration inselection of an element designed for incorporation in a regeneratorsystem and cannot be avoided. Electroless nickel, however, can bestress relieved at 600 - 7000F. Additional samples were thereforefabricated and stress relieved at 650*F for 30 minutes, prior to ex-posure. The electroless nickel plate on these samples was still intactafter 5CO hours exposure at 12000F.

5-3

WRIGHT AERONAUTICAL DIVISION " CURTISS WRIGHT CORPORATION * WOOD RIDGE I, i ; A

Metallographic examination of samples exposed at 1200"F revealed whatappeared to be a eutectic Cu3 P phase. This phase formed as a resultof the diffusion of phosphorous, inherently present in the electrolessnickel plate (8%), into the copper at the exposure temperature. Expo-sure at a lower temperature (900*F) for an identical period of time,significantly reduced the formation of the subject phase (Fig. 5-8).In an attempt to inhibit the diffusion of phosphorous into the coppera "flash" of electroless gold was applied to the samples prior to thenickel. Exposure of these samples revealed that the intermediate goldcoating affected the rate of Cu3 P precipitation, but did not prevent it.

The effect of the precipitated Cu3 P phase on the thermal efficiency ofthe fin is indeterminate and actual measurements would have to be takenif the electroless nickel coating were to be considered for use in aliquid metal regenerator. Examination of a portion of the copper-phos-phorous phase diagram (Fig. 5-9) suggests that eutectic melting willoccur if this system is subjected to an operating temperature in thevicinity of 1300*F.

5-4

WRIGHT AERONAUTICAL DIVISION - CURTfSS.WRIGHT CORPORATION " WOOD-RIDGE N I U S A

ANNEALING OF 0.003" ALUMINUM BRONZE (5%) STRIP

Heat Treatment Test No. Elongation in 6"

In.

As Received 1 0.125 2.1

2 0.18 3.0

950 0 F, 5 min. 1 1.125 18.8

2 1.5 25

12000 F, 5 min. 1 2.75 46

2 2.75 46

17000 F, 5 min. 1 1.69 28.2

2 1.75 29.2

Note: All heat treatments performed in Argon and air cooled.

Table 5-1

WRIGHT AERONAUTICAL DIVISION • CURTISS-WRIGHT CORPORATION - WOOD-RIDGE, N IJ S A

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WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE NJ U SA

PIN 50677 Hag. 200x PN 50678 Hag. 200xAs Rolled Exposed 112 hours

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5529 Figure 5-3

WRIGHT AERONAUTICAL DIVISION - CURTISS WRIGHT CORPORATION - WOOD-RIDGE N J 1 A

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5521 Figure 5-5

WRIGHT AERONAUTICAL DIVýSIUN - CURTISS-WRIGHT CORPORATION - WOOD) RIDGE N J A

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"'C Figure 5-6

WRIGHT AERONAUITICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE U SA

FOCI 3 Etch Mag. 200X

Cracking of the eleotroless nickel plate after 15 min.exposure at 12000F.

5527 Figure 5-7

WRIGHT AERONALITICAL DIVISION - CURJISý WRI(,HT CORPORATION - WOOD-R,[LGF [ A

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Photomicrographs depicting the formation of Cu P at theelectroliess nickel - OFHC copper interface. ;9te thedecrease in the amount of phase present and the absenceof apparent eutectic melting at the lover exposuretemperature (9000F).

5526 Figure 5-8

WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE N 1

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WRIGHT AERONALITICAL DIVISION C CURTISb WRlI', T tC .I PORAIION • WOOD Fq1DGF - A

VI FABRICATION STUDIES

Currently available heat transfer elements in sizes applicable toflight type heat exchangers are produced by means of a spiral windingfin material on tubes. The most promising alternate method of fabrica-tion yields a disc fin type heat transfer element. Fabrication of discfin elements involved developing special manufacturing processes formounting and bonding large numbers of closely spaced, small diameter,thin fins onto tubing. One of the major difficulties encountered inmaking a heat transfer element of this type was the handling of the"foil-like" fins without distortion and maintaining the spacing of suchfins until they were bonded to the tube. On the other hand, the designhad the distinct advantage of being particularly well suited to lowductility fin materials and to thin wall tubing, since it did not dis-tort the tube wall. The design further allowed wide latitude in theratio of fin diameter to tube diameter. The potential advantages ofdisc fin elements led to an investigation of production methods toposition fins. The inclusion of temporary spacer material between finswas selected as the most promising approach.

A. Spacer Materials

The function of the spacer material in the fabrication of disc fin typeheat transfer elements was to maintain the uniform spacing of the finsuntil they were bonded to the tube. Spacers of this nature may beclassified in the following three categories:

1. Leachable: This type of spacer would be in the form of a "washer",fabricated from a leachable ceramic material. The spacers wouldbe leached out after brazing.

2. Mechanical: A spacer of this nature would be "V' shaped, fabricatedfrom a metallic or ceramic material. The spacers would be removedafter brazing and could be used a number of times.

3. Expendable: Expendable spacers would also be "washer" shaped andcould be fabricated from a number of materials (i.e., cardboard,fiberboard, plastic, etc.). They would disintegrate during thebrazing operation leaving little or no residue.

Preliminary investigation of the three types of spacer materials indi-cated that the cost of utilizing leachable or mechanical spacers wouldbe excessive. Both the initial material costs and the subsequenthandling costs incurred by utilizing materials of this nature would beprohibitive. Because of the inherent disadvantages associated with theleachable and mechanical spacers, effort was concentrated on the use ofexpendable spacer materials.

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WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION * WOOD-RIOGE. N S A

Initial brazing trials were conducted utilizing 0.005" thick x 0.4375"O.D. aluminum bronze and OFHC copper fins. Cardboard, fiberboard, andasbestos spacer materials were evaluated. Of this group, fiberboardspacers yielded the most promising finned tube sample, leaving a loosecarbonaceous deposit after brazing. This deposit was easily removedby washing with water and a fine bristle brush, leaving the fins onlyslightly discolored.

It was suggested that thermoplastic materials would vaporize and/orburn off more readily at the brazing temperatures. In addition, theywould be easier to handle and bond to the fin material, especially inproduction quantities. "Teflon", polyethelene, and "Delrin" wereselected for evaluation. All the plastic materials appeared to producefinned tube samples with low fin distortion and good fin alignment.The light ash remaining after brazing was easily blown away with an airhose. Fin discoloration was evident, and attempts to clean the tubesutilizing mechanical and chemical techniques were unsuccessful. Itwas believed that this discoloration was produced by some type ofreaction between the brazing flux and the spacer material as it meltedand vaporized during the brazing cycle. The discoloration was foundto be shallow and will have no effect on heat transfer. To furtherevaluate the production capabilities of the three plastic spacermaterials, several sheet samples were made consisting of 0.003" thickaluminum bronze sheet bonded to plastic sheet with a commercial adhesive.These samples were then processed through the fin stamping dies to de-termine the feasibility of stamping the bonded sheet. With the poly-ethelene-aluminum bronze sample the plastic proved to be too soft andit was very difficult to produce a clean cut. This condition led tofouling of the dies, making it inadvisable to use this material in aproduction set up. Difficulty was also encountered with the sampleincorporating the Teflon sheet. This plastic inhibited the formationof the "flanges" on the I.D. of the fin. Brazing trials indicatedthat without these flanges it was impossible to maintain the desiredfin alignment and joint efficiency. The Delrin-aluminum bronze sample,on the other hand, did not present any problems during the stampingoperation. On the basis of this evaluation, brazing trials were con-tinued utilizing fins stamped from 0.003" thick aluminum bronze sheetbonded to 0.030P Delrin sheet.

B. Brazina Development

Initial brazing trials for the evaluation of spacer materials were per-formed using Handy & Harman "Braze ETX" and H & H type A-1 flux. Thebraze was a coumercial silver solder especially recommended for alumin-um bronze. The samples were in the form of 0.1875" O.D. stainlesssteel tubes approximately one inch long. The braze was preplaced onthe tube by means of a "tinning" operation with an acetylene torch.

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WRIGHT AERONAUTICAL DIVI5ION - CUR VISS-WRIGHT CORPORATION - WOOD RIDGE N S A

The fins were then placed on the tube with the aid of spacers and theentire assembly was inserted into a hydrogen atmosphere furnace.

The "timring!' method of applying the braze alloy was adequate for smalllaboratory samples but could not be adopted as a production process.The coating produced was inconsistent in thickness, especially ifapplied to long pieces. The following methods were, therefore, devisedto coat the tubing with a silver-copper eutectic braze alloy:

1. The braze alloy was procured in fine power form, mixed with a flux,and painted on the tube along the desired length. The preparedtube was then run through an induction coil which melted and flowedthe braze alloy on the tube. Refinement of this technique has pro-duced uniform coatings approximately 0.001 inch thick.

2. Silver and copper were electrodeposited separately on the tube tothe desired thickness. The amount of each constituent was con-trolled so that the final coating contained approximately theeutectic composition of 707. silver and 307. copper. Fins wereplaced on the tube and the assembly was heated to 14500F for 10-15minutes. The two plates alloyed and eutectic melting resulted,which brazed the fins to the tube. Brazing primarily occurredunder the flanges on the I.D. of the fin (Fig.6-1). Of the twomethods, electrodeposition was preferable because it was consideredto be adaptable as a batch process and easier to control.

In addition to the Ag-Cu plating and brazing technique described above,samples were procured with a gold-copper eutectic alloy plate. Interestwas expressed in this alloy as a high temperature, oxidation-corrosionresistant braze material. The Au-Cu composite plate required less timeat the brazing temperature and samples were successfully brazed in 5-7minutes at 17500F. The effect of the higher brazing temperature on thecorrosion resistance of the aluminum bronze fin material has not beendetermined at the present time and further investigation would bewarranted.

Gold-nickel alloy systems may also be of interest for brazing finnedtubes but application of this alloy to the tube may be difficult un-less cladding techniques can be applied.

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WRIGHT AERONAUTICAL DI0I1!ION " CURTISS-WRIGHT CORPORATION " WOOD-RIDGE 1` J U S A

The fins were then placed on the tube with the aid of spacers and theentire assembly was inserted into a hydrogen atmosphere furnace.

The "tinning" method of applying the braze alloy was adequate for smalllaboratory samples but could not be adopted as a production process.The coating produced was inconsistent in thickness, especially ifapplied to long pieces. The following methods were, therefore, devisedto coat the tubing with a silver-copper eutectic braze alloy:

1. The braze alloy was procured in fine power form, mixed with a flux,and painted on the tube along the desired length. The preparedtube was then run through an induction coil which melted and flowedthe braze alloy on the tube. Refinement of this technique has pro-duced uniform coatings approximately 0.001 inch thick.

2. Silver and copper were electrodeposited separately on the tube tothe desired thickness. The amount of each constituent was con-trolled so that the final coating contained approximately theeutectic composition of 707. silver and 307 copper. Fins wereplaced on the tube and the assembly was heated to 1450*F for 10-15minutes. The two plates alloyed and eutectic melting resulted,which brazed the fins to the tube. Brazing primarily occurredunder the flanges on the I.D. of the fin (Fig.6-1). Of the twomethods, electrodeposition was preferable because it was consideredto be adaptable as a batch process and easier to control.

In addition to the Ag-Cu plating and brazing technique described above,samples were procured with a gold-copper eutectic alloy plate. Interestwas expressed in this alloy as a high temperature, oxidation-corrosionresistant braze material. The Au-Cu composite plate required less timeat the brazing temperature and samples were successfully brazed in 5-7minutes at 17500F. The effect of the higher brazing temperature on thecorrosion resistance of the aluminum bronze fin material has not beendetermined at the present time and further investigation would bewarranted.

Gold-nickel alloy systems may also be of interest for brazing finnedtubes but application of this alloy to the tube may be difficult un-less cladding techniques can be applied.

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WRIGHT AERONALJTICAL DIVISION ° CURTISS-WRIGHT CORPORATION - WOOD-RIDGE N I U S A

C. Spiral Winding Studies

Current materials and fabrication techniques, employed in the manufac-ture of spirally wound finned tubes, imposed substantial limitationson the performance versus weight of liquid metal systems utilizingthese elements. The primary problem areas were the following:

(1) The steady decrease in ductility, incurred as the thicknessof the fin material (stainless clad copper) was progressivelyreduced ,which prevented the use of this fin material in thick-nesses less than 0.005".

(2) Grooving of the tube to position the fins resulted in dis-tortion of the tube wall. The amount of distortion allowabledictated the wall thickness of the tubing. Tube distortionincreases the resistance to the flow of liquid metal,andtherefore, it undesirable.

(3) The stainless clad copper fin material required protection ofthe edges from the corrosive and oxidizing environmentsencountered during service. Protection was accomplished byspraying the braze material on the finned tubes, but resultedin an additional weight penalty.

Investigations to reduce these problem areas were severely limitedbecause available capabilities had to be devoted to production of heattransfer elements used in the fabrication of the heat exchangersevaluated under a separate and concurrent investigation of liquidmetal regenerator systems. However, sufficient work was performed todemonstrate the feasibility of using aluminum bronze as a fin materialto overcome the limitations described above.

Sample finned tube elements have been successfully made by winding0.003" thick aluminum bronze fin stock, in the annealed condition,without the aid of positioning grooves. Methods of bonding the spiralwound aluminum bronze fins by brazing were not investigated becauseof cost and time limitations of this program. Precoating the tubeswith braze alloy(as was demonstrated in the disc fin investigation)appears to be a logical approach,as well as the application of abraze material,in wire form, spiral wound between the fins.

Spiral wound fins made by convoluting the inner edge of the fin materialwere not investigated because current manufacturing practice did notproduce tubes of appropriate size. There was also some concern as tothe susceptibility of such fins to fouling. Recent experience withhigh velocity gas streams indicated that fouling may not be a problem.Further consideration of convoluted finned tubes is warranted.

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WRI GHýIT AtCR JNA 'I AL DIV I. 1r)N * tHRTIHH WRICHT C 1HF1P HIRAT 1)N ° IV -1 [.

It is expected that convoluted finned tubes could give higher perform-ance than flat wound fins when compared on an equal weight basisbecause of higher gas heat transfer rates. Figure 7-2 depicts acopper fin tube of this type in the smallest size currently available(.250 tube). Discussion with manufacturers of such finned tubesindicate the possibility of going to smaller diameter tubes and closerfin spacing. The material and brazing techniques developed under thisprogram appear to be applicable to this type of tubing. Thesusceptibility to fouling under engine conditions must be investigated.

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VII FABRICATION OF SAMPLE HEAT EXCHANGER

In order to prove the developed fabrication technique on a semi-produc-tion basis a sample 6 in x 6 in x4 in single pass heat exchanger wasfabricated employing 0.003" thick x 0.375" O.D. 57. aluminum bronzefins, with a fin spacing of 30 per inch along a 6" tube length. Thestainless steel tube wall thickness was reduced from the current0.015" to 0.012". The silver-copper plating and brazing technique,described in the previous section, was used in the fabrication of themajority of the finned tubing. Approximately one-quarter of the tubesin the heat exchanger consisted of copper clad stainless steel tubingwith 0.001" of silver plated over the cladding. No significantdifference in brazing behavior was noted between the two methods ofbraze application.

Aluminum bronze strip (0.003" thick) was bonded to 0.030" thick Delrinplastic strip, using a double adhesive tape. Fins were then stampedfrom this composite and subsequently stacked on the braze coatedtubing. A total plating thickness of approximately 0.0025" was usedfor this application. The stacked tubes were then brazed in a dis-sociated ammonia atmosphere furnace for 15 minutes at 1450-1500"F.The carbonaceous deposit remaining after brazing was blown away withan air hose and the tubes were mechanically cleaned by brushing witha fine-bristled brush and warm water in a direction parallel to thefins. Figure 7-1 shows a typical finned tube element prior tobrazing (foreground), and after the cleaning operation. The brazedelements (99 total) were then assembled, expanded, and welded intoheaders. It should be noted that no difficulties were encountered ineffecting a sound tube-to-header joint with the 0.012" wall tubing.Figure 7-2 depicts the assembled and welded heat exchanger. Theuse of 0.012" wall tubing instead of 0.015" wall tube produces afurther 97. weight reduction in the total finned tube weight. Thinnerwall tubes are feasible on the basis of tinning techniques and lowerinternal pressure loads.

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WRIGHT AERONAUTICAL DIVISION " CURTISS-WRIGHT CORPORATION - WOOD-RIDGE. N. J.. U. S. A.

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WRI.ý,ff A Rý,%~ I AL OIDI ,ION - C LIRTISS.WHIGHT CORPORATION *WO)OD-RIDGE N I U SA

VIII RESULTS & DISCUSSION

Demonstration quantities of finned tube heat transfer elements forliquid metal heat exchangers have been made which are 307 lighter thanthose currently available when compared on the basis of equal sizeand equal performance. Further weight savings, based on equal perform-ance, ave possible by changes in geometry and in size of the heattransfer elements. These gains in performance are derived frommetallurgical, analytical, and fabrication findings described below.

(1) Metallurgical investigations have demonstrated that aluminupibronze (5% aluminum, 95% copper) has good oxidation resis-tance at operating temperatures likely to be encountered inliquid metal regenerator systems for advanced turbopropengines. This property plus its relatively high thermalconductivity at operating conditions makes aluminum bronzean excellent fin material for finned tube heat transferelements of liquid metal heat exchangers.

(2) The high ductility of aluminum bronze obtained by heattreatment has made possible the forming of this materialinto spiral wound fins of thicknesses and diameters appropri-ate for high performance finned tube heat transfer elements.

(3) Aluminum bronze can be bonded to stainless steel tubes bysilver-copper, gold-copper, and gold-nickel braze alloys,of eutectic composition.

(4) Optimization studies show that maximum performance versusweight is gained by utilizing the thinnest fins that can befabricated.

(5) The heat transfer characteristics of the heat exchangersindicate optimum performance versus weight is obtained bya 2:1 fin diameter to tube diameter ratio in the compressorexchanger, and a 2.5:1 diameter to tube ratio in the turbineexchanger for sizes considered most practical at this time.

(6) Grooves for positioning the fins can be eliminated in spiralwound finned tubes allowing the use of thinner wall tubeswithout incurring distortion in the tube wall.

(7) A new fabrication method for manufacturing disc fin tubes byuse of expendable spacers for fin alignment and spacingduring the brazing operation has been developed and appearseconomical.

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A D I VA Iv f ) r, P T-N I W H I ý, , K.'' 1

(8) The new fabrication method imposes no limits on tube wallthickness. Therefore, the thinnest wall tube can be usedconsistent with operating conditions.

(9) The new fabrication method imposes no limits on fin to tubediameter ratio. Therefore, further optimization of heattransfer elements for each heat exchanger can be madeconsistent with liquid metal pumping requirements.

(10) The new fabrication method imposes no ductility requirements.Therefore, fin materials formerly considered impracticalbecause of low ductility, such as .003" stainless steelclad copper, are feasible.

(11) Significant weight reduction for equal performance can beobtained by increasing the number of fins per inch of tubefrom 30 to a maximum yet to be determined by fouling tests.

(12) Electroless nickel plating provides durable oxidation pro-tection for copper fins and can he applied in uniformlythin layers on finned tubes. However, the migration ofphosphorus, inherent in the plating process, may have adetrimental effect on the thermal conductivity of the fins.

(13) Vapor deposited andi electrolytic coatings investigated wereunsatisfactory for protecting copper fins. However, thebroad scope of this field and possible refinements intechniques warrant further efforts before conclusions can bemade on coated fins.

(14) A practical fabrication technique for making thin taperedfins was not conceived during this investigation.

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WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE, N J. U. S. A.

Ix MgECOEKNTIONS

The limited scope of this study has made necessary the omission ofseveral areas of investigation considered relative to f'.rther increas-ins performance versus weight of the heat transfer elements. Alsoefforts to determine practical limits of sizes and materials underactual service conditions have been restricted. In view of the poten-tial of liquid metal regenerators for turboprop and turboshaft enginesand the vital function of the heat transfer elements in such systemsthe following recommendations are made.

(1) Continue investigations of braze materials for bonding finsto tubes.

(2) Investigate brazing techniques for bonding spiral wound finsto tubes.

(3) Fabricate full length disc fin tube serpentines for manufac-turing and performance evaluation.

(4) Operate liquid metal loops containing advonced heat transferelements under engine conditions.

(5) Investigate the practical limits of fin spacing by longterm fouling tests.

(6) Investigate improvements in heat transfer performance byconvoluted fins and determine their susceptibility tofouling.

(7) Investigate the effect of phosphorus migration from electro-less nickel plating on the thermal conductivity of copperfins.

(8) Conduct further investigations on protective coatings forcopper fins.

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WRIGHT AERONAUTICAL DIVISION - CURTISS-WRIGHT CORPORATION - WOOD-RIDGE. N. J.. U. S. A.